Effects of porosity on the measured fracture energy of brittle materials

Abstract Although it is known that growing cracks will interact with pores, it is unclear whether the magnitude of this effect is sufficient to influence the fracture energy. To study this, experiments have been carried out where cracks have been grown through simple distributions of pores in poly(methyl methacrylate). These show that the applied force required to grow the crack between two pores can be greater than that required to grow the crack in the pore-free material. Direct observation during crack growth shows that this increase in applied force is associated with the crack front becoming curved. Based on these observations, the effect of equiaxed pores on the fracture energy of brittle materials has been quantitatively described. The analysis predicts how the relative fracture energy should be influenced by the pore volume fraction, and that it should be independent of the size of the pores or the fracture energy of the matrix. These predictions give good agreement with experimental measurements in different ceramic materials, in which the microstructure of the matrix surrounding the pores does not change with pore volume fraction.